17. Between the Proreptilia and the Mammalia, which latter occur in the Upper Triassic epoch, we have necessarily to intercalate a group of very low reptiles, which are still so generalized that their descendants could branch off either into the Reptilia proper or into the Mammalia. The changes concerned chiefly the brain and the heart; of the skeleton, the skull and the pelvis; and, of the tegumentary structures, the formation of a hairy covering. Many such creatures existed in the Triassic epoch--namely, the _Theromorpha_--some of which indeed possess so many characteristics which otherwise occur in the Mammalia only, that these creatures have been termed _Sauro-Mammalia_. However, it has to be emphasized that none of the Theromorpha hitherto discovered fulfils all the requirements which would entitle them to this important linking position. They only give us an approximate idea of what this link was like. 18. Stage of the _Promammalia_, or _Prototheria_. The only surviving members are the famous duck-bill, Ornithorhynchus, and the spiny ant-eaters, Echidna and Proechidna, of the Australian region. These few genera, however, differ so much from one another in various important respects that they cannot but be remnants of an originally much larger group. Indeed, many fossils from the Upper Triassic and from the Jurassic strata have without much doubt to be referred to the Prototheria. The Prototheria are typical mammals, because they possess the following characteristics: The heart is completely quadrilocular; the blood is warm, and its red corpuscles have, owing to the loss of their nucleus, been modified from biconvex into biconcave discs; they have a hairy coat and sweat glands, and two occipital condyles; the ilio-sacral connection is preacetabular; the ankle-joint is cruro-tarsal; the quadrate bone of the Reptilia has ceased to carry the under jaw, which now articulates directly with the squamosal portion of the skull. Their low position is shown by the retention of the following reptilian features: Complete coracoid bones and a T-shaped interclavicle; a cloaca, or common chamber for the passage of the fæces, the genital and the urinary products; they are still oviparous; the embryo develops without a chorion, and is therefore not nourished through a placenta. Even the milk glands, which are absolutely peculiar to the Mammalia, are still in a very primitive stage, and do not yet produce milk proper; and there is only a temporary shallow marsupium.